An ON/OFF valve (duty valve) such that it is repeatedly completely opened and completely closed at a high rate to control a fluid pressure by a duty ratio thereof is known as a fluid pressure regulating valve that regulates a fluid pressure supplied to a transmission clutch operated by a fluid pressure.
A drive circuit of such a duty valve is mainly configured by a power source, a transistor provided between the power source and the duty valve, and a CPU that ON-OFF drives the transistor.
The CPU has a voltage monitor terminal connected between the transistor and the duty valve, and a voltage supplied to the duty valve is detected by the voltage monitor terminal.
For example, when a transmission clutch is completely disconnected, the CPU performs the all-time on drive of the transistor so as to close the duty valve completely.
In this case, when no fault occurs e.g. in a wiring connected to the transistor and duty valve, a predetermined voltage is detected by the voltage monitor terminal, but when the wiring is ground short circuited, the voltage detected by the voltage monitor terminal is 0 V although the transistor is ON driven by the CPU.
Accordingly, in the drive circuit of the duty valve, a ground short fault is determined if the detection voltage of the voltage monitor terminal is zero when the transmission clutch is completely disconnected (the CPU performs the all-time ON drive).
In recent years, a linear solenoid valve whose opening degree can be changed continuously has been used instead of the duty valve in order to increase the resolution of fluid pressure of the transmission clutch (see, for example, Japanese Patent Application Laid-open No. H03-199757, Japanese Patent Application Laid-open No. H07-119816 and Japanese Patent Application Laid-open No. H02-180357).